Im doing the exact same scenario, only on a 35 which is basically the same as your car except for the grille. I have looked around and have discovered that the ribs which are the supporting system for the sheetmetal are the most important part of this problem. If you fabricate them properly and at the right angle, the sheet metal will automatically lay right. It doesen't matter if the sheet metal came from another car roof or you just buy it, it is thin and will conform to the ribs. I believe therefore, that the ribs are the key element.